Lithium-based active materials and preparation thereof

ABSTRACT

The invention provides novel lithium-mixed metal materials which, upon electrochemical interaction, release lithium ions, and are capable of reversibly cycling lithium ions. The invention provides a rechargeable lithium battery which comprises an electrode formed from the novel lithium-mixed metal materials. Methods for making the novel lithium-mixed metal materials and methods for using such lithium-mixed metal materials in electrochemical cells are also provided. The lithium-mixed metal materials comprise lithium and at least one other metal besides lithium. Preferred materials are lithium-mixed metal phosphates which contain lithium and two other metals besides lithium.

FIELD OF THE INVENTION

This invention relates to improved materials usable as electrode activematerials and to their preparation.

BACKGROUND OF THE INVENTION

Lithium batteries are prepared from one or more lithium electrochemicalcells containing electrochemically active (electroactive) materials.Such cells typically include an anode (negative electrode), a cathode(positive electrode), and an electrolyte interposed between spaced apartpositive and negative electrodes. Batteries with anodes of metalliclithium and containing metal chalcogenide cathode active material areknown. The electrolyte typically comprises a salt of lithium dissolvedin one or more solvents, typically nonaqueous (aprotic) organicsolvents. Other electrolytes are solid electrolytes typically calledpolymeric matrixes that contain an ionic conductive medium, typically ametallic powder or salt, in combination with a polymer that itself maybe tonically conductive which is electrically insulating. By convention,during discharge of the cell, the negative electrode of the cell isdefined as the anode. Cells having a metallic lithium anode and metalchalcogenide cathode are charged in an initial condition. Duringdischarge, lithium ions from the metallic anode pass through the liquidelectrolyte to the electrochemical active (electroactive) material ofthe cathode whereupon they release electrical energy to an externalcircuit.

It has recently been suggested to replace the lithium metal anode withan insertion anode, such as a lithium metal chalcogenide or lithiummetal oxide. Carbon anodes, such as coke and graphite, are alsoinsertion materials. Such negative electrodes are used withlithium-containing insertion cathodes, in order to form an electroactivecouple in a cell. Such cells, in an initial condition, are not charged.In order to be used to deliver electrochemical energy, such cells mustbe charged in order to transfer lithium to the anode from thelithium-containing cathode. During discharge the lithium is transferredfrom the anode back to the cathode. During a subsequent recharge, thelithium is transferred back to the anode where it re-inserts. Uponsubsequent charge and discharge, the lithium ions (Li⁺) are transportedbetween the electrodes. Such rechargeable batteries, having no freemetallic species are called rechargeable ion batteries or rocking chairbatteries. See U.S. Pat. Nos. 5,418,090; 4,464,447; 4,194,062; and5,130,211.

Preferred positive electrode active materials include LiCoO₂, LiMn₂O₄,and LiNiO₂. The cobalt compounds are relatively expensive and the nickelcompounds are difficult to synthesize. A relatively economical positiveelectrode is LiMn₂O₄, for which methods of synthesis are known. Thelithium cobalt oxide (LiCoO₂), the lithium manganese oxide (LiMn₂O₄),and the lithium nickel oxide (LiNiO₂) all have a common disadvantage inthat the charge capacity of a cell comprising such cathodes suffers asignificant loss in capacity. That is, the initial capacity available(amp hours/gram) from LiMn₂O₄, LiNiO₂, and LiCoO₂ is less than thetheoretical capacity because significantly less than 1 atomic unit oflithium engages in the electrochemical reaction. Such an initialcapacity value is significantly diminished during the first cycleoperation and such capacity further diminishes on every successive cycleof operation. For LiNiO₂ and LiCoO₂ only about 0.5 atomic units oflithium is reversibly cycled during cell operation. Many attempts havebeen made to reduce capacity fading, for example, as described in U.S.Pat. No. 4,828,834 by Nagaura et al. However, the presently known andcommonly used, alkali transition metal oxide compounds suffer fromrelatively low capacity. Therefore, there remains the difficulty ofobtaining a lithium-containing electrode material having acceptablecapacity without disadvantage of significant capacity loss when used ina cell.

SUMMARY OF THE INVENTION

The invention provides novel lithium-mixed metal materials which, uponelectrochemical interaction, release lithium ions, and are capable ofreversibly cycling lithium ions. The invention provides a rechargeablelithium battery which comprises an electrode formed from the novellithium-mixed metal materials. Methods for making the novellithium-mixed metal materials and methods for using such lithium-mixedmetal materials in electrochemical cells are also provided. Thelithium-mixed metal materials comprise lithium and at least one othermetal besides lithium. Preferred materials are lithium-mixed metalphosphates which contain lithium and two other metals besides lithium.Accordingly, the invention provides a rechargeable lithium battery whichcomprises an electrolyte; a first electrode having a compatible activematerial; and a second electrode comprising the novel materials. In oneaspect, the novel materials are lithium-mixed metal phosphates whichpreferably used as a positive electrode active material, reversiblycycle lithium ions with the compatible negative electrode activematerial. Desirably, the lithium-mixed metal phosphate is represented bythe nominal general formula Li_(a)MI_(b)MII_(c)(PO₄)_(d). Such compoundsinclude Li₁MI_(a)MII_(b)PO₄ and Li₃MI_(a)MII_(b)(PO₄)₃; therefore, in aninitial condition 0≦a≦1 or 0≦a≦3, respectively. During cycling, xquantity of lithium is released where 0≦x≦a. In the general formula, thesum of b plus c is up to about 2. Specific examples areLi₁MI_(1-y)MII_(y)PO₄ and Li₃MI_(2-y)MII_(y)(PO₄)₃.

In one aspect, MI and MII are the same. In a preferred aspect, MI andMII are different from one another. At least one of MI and MII is anelement capable of an oxidation state higher than that initially presentin the lithium-mixed metal phosphate compound. Correspondingly, at leastone of MI and MII has more than one oxidation state in the phosphatecompound, and more than one oxidation state above the ground state M⁰.The term oxidation state and valence state are used in the artinterchangeably.

In another aspect, both MI and MII may have more than one oxidationstate and both may be oxidizable from the state initially present in thephosphate compound. Desirably, MII is a metal or semi-metal having a +2oxidation state, and is selected from Groups 2, 12 and 14 of thePeriodic Table. Desirably, MII is selected from non-transition metalsand semi-metals. In one embodiment, MII has only one oxidation state andis nonoxidizable from its oxidation state in the lithium-mixed metalcompound. In another embodiment, MII has more than one oxidation state.Examples of semi-metals having more than one oxidation state areselenium and tellurium; other non-transition metals with more than oneoxidation state are tin and lead. Preferably, MII is selected from Mg(magnesium), Ca (calcium), Zn (zinc), Sr (strontium), Pb (lead), Cd(cadmium), Sn (tin), Ba (barium), and Be (beryllium), and mixturesthereof. In another preferred aspect, MII is a metal having a +2oxidation state and having more than one oxidation state, and isoxidizable from its oxidation state in lithium-mixed metal compound.

Desirably, MI is selected from Fe (iron), Co (cobalt), Ni (nickel), Mn(manganese), Cu (copper), V (vanadium), Sn (tin), Ti (titanium), Cr(chromium), and mixtures thereof. As can be seen, MI is preferablyselected from the first row of transition metals and further includestin, and MI preferably initially has a +2 oxidation state.

In a preferred aspect, the product LiMI_(1-y)MII_(y)PO₄ is an olivinestructure and the product Li₃MI_(1-y)(PO₄)₃ is a rhombohedral ormonoclinic Nasicon structure. In another aspect, the term “nominalformula” refers to the fact that the relative proportion of atomicspecies may vary slightly on the order of 2 percent to 5 percent, ormore typically, 1 percent to 3 percent. In still another aspect, anyportion of P (phosphorous) may be substituted by Si (silicon), S(sulfur), and/or As (arsenic); and any portion of O (oxygen) may besubstituted by halogen, preferably F (fluorine). These aspects are alsodisclosed in U.S. patent application Ser. No. 09/105,748 filed Jun. 26,1998, and Ser. No. 09/274,371 filed Mar. 23, 1999; and in U.S. Pat. No.5,871,866 issued Feb. 16, 1999, which is incorporated by reference inits entirety; each of the listed applications and patents are co-ownedby the assignee of the present invention.

The metal phosphates are alternatively represented by the nominalgeneral formulas such as Li_(1-x)MI_(1-y)MII_(y)PO₄(0≦×≦1), andLi_(3-x)MI_(2-y)MII_(y)(PO₄)₃ signifying capability to release andreinsert lithium. The term “general” refers to a family of compounds,with M, x and y representing variations therein. The expressions 2-y and1-y each signify that the relative amount of MI and MII may vary. Inaddition, as stated above, MI may be a mixture of metals meeting theearlier stated criteria for MI. In addition, MII may be a mixture ofmetallic elements meeting the stated criteria for MII. Preferably, whereMII is a mixture, it is a mixture of 2 metallic elements; and where MIis a mixture, it is a mixture of 2 metals. Preferably, each such metaland metallic element has a +2 oxidation state in the initial phosphatecompound.

The active material of the counter electrode is any material compatiblewith the lithium-mixed metal phosphate of the invention. Where thelithium-mixed metal phosphate is used as a positive electrode activematerial, metallic lithium, lithium-containing material, ornon-lithium-containing material may be used as the negative electrodeactive material. The negative electrode is desirably a nonmetallicinsertion material. Desirably, the negative electrode comprises anactive material from the group consisting of metal oxide, particularlytransition metal oxide, metal chalcogenide, carbon, graphite, andmixtures thereof. It is preferred that the anode active materialcomprises a carbonaceous material such as graphite. The lithium-mixedmetal phosphate of the invention may also be used as a negativeelectrode material.

In another embodiment, the present invention provides a method ofpreparing a compound of the nominal general formulaLi_(a)MI_(b)MII_(c)(PO₄)_(d) where 0<a≦3; the sum of b plus c is greaterthan zero and up to about 2; and 0<d≦3. Preferred compounds includeLi₃MI_(b)MII_(c)(PO₄)₃ where b plus c is about 2; and LiMI_(b)MII_(c)PO₄where b plus c is about 1. The method comprises providing startingmaterials in particle form. The starting (precursor) materials include alithium-containing compound, one or more metal containing compounds, acompound capable of providing the phosphate (PO₄)⁻³ anion, and carbon.Preferably, the lithium-containing compound is in particle form, and anexample is lithium salt. Preferably, the phosphate-containing anioncompound is in particle form, and examples include metal phosphate saltand diammonium hydrogen phosphate (DAHP) and ammonium dihydrogenphosphate (ADHP). The lithium compound, one or more metal compounds, andphosphate compound are included in a proportion which provides thestated nominal general formula. The starting materials are mixedtogether with carbon, which is included in an amount sufficient toreduce the metal ion of one or more of the metal-containing startingmaterials without full reduction to an elemental metal state. Excessquantities of carbon and one or more other starting materials (i.e., 5to 10% excess) may be used to enhance product quality. A small amount ofcarbon, remaining after the reaction, functions as a conductiveconstituent in the ultimate electrode formulation. This is an advantagesince such remaining carbon is very intimately mixed with the productactive material. Accordingly, large quantities of excess carbon, on theorder of 100% excess carbon are useable in the process. The carbonpresent during compound formation is thought to be intimately dispersedthroughout the precursor and product. This provides many advantages,including the enhanced conductivity of the product. The presence ofcarbon particles in the starting materials is also thought to providenucleation sites for the production of the product crystals.

The starting materials are intimately mixed and then reacted togetherwhere the reaction is initiated by heat and is preferably conducted in anonoxidizing, inert atmosphere, whereby the lithium, metal from themetal compound(s), and phosphate combine to form theLi_(a)MI_(b)MII_(c)(PO₄)_(d) product. Before reacting the compounds, theparticles are intermingled to form an essentially homogeneous powdermixture of the precursors. In one aspect, the precursor powders aredry-mixed using a ball mill, such as zirconia media. Then the mixedpowders are pressed into pellets. In another aspect, the precursorpowders are mixed with a binder. The binder is selected so as to notinhibit reaction between particles of the powders. Therefore, preferredbinders decompose or evaporate at a temperature less than the reactiontemperature. Examples include mineral oils (i.e., glycerol, or C-18hydrocarbon mineral oil) and polymers which decompose (carbonize) toform a carbon residue before the reaction starts, or which evaporatebefore the reaction starts. In still another aspect, intermingling isconducted by forming a wet mixture using a volatile solvent and then theintermingled particles are pressed together in pellet form to providegood grain-to-grain contact.

Although it is desired that the precursor compounds be present in aproportion which provides the stated general formula of the product, thelithium compound may be present in an excess amount on the order of 5percent excess lithium compared to a stoichiometric mixture of theprecursors. And the carbon may be present at up to 100% excess comparedto the stoichiometric amount. The method of the invention may also beused to prepare other novel products, and to prepare known products. Anumber of lithium compounds are available as precursors, such as lithiumacetate (LiOOCCH₃), lithium hydroxide, lithium nitrate (LiNO₃), lithiumoxalate (Li₂C₂O₄), lithium oxide (Li₂O), lithium phosphate (Li₃PO₄),lithium dihydrogen phosphate (LiH₂PO₄), lithium vanadate (LiVO₃), andlithium carbonate (Li₂CO₃). The lithium carbonate is preferred for thesolid state reaction since it has a very high melting point and commonlyreacts with the other precursors before melting. Lithium carbonate has amelting point over 600° C. and it decomposes in the presence of theother precursors and/or effectively reacts with the other precursorsbefore melting. In contrast, lithium hydroxide melts at about 400° C. Atsome reaction temperatures preferred herein of over 450° C. the lithiumhydroxide will melt before any significant reaction with the otherprecursors occurs to an effective extent. This melting renders thereaction very difficult to control. In addition, anhydrous LiOH ishighly hygroscopic and a significant quantity of water is releasedduring the reaction. Such water needs to be removed from the oven andthe resultant product may need to be dried. In one preferred aspect, thesolid state reaction made possible by the present invention is muchpreferred since it is conducted at temperatures at which thelithium-containing compound reacts with the other reactants beforemelting. Therefore, lithium hydroxide is useable as a precursor in themethod of the invention in combination with some precursors,particularly the phosphates. The method of the invention is able to beconducted as an economical carbothermal-based process with a widevariety of precursors and over a relatively broad temperature range.

The aforesaid precursor compounds (starting materials) are generallycrystals, granules, and powders and are generally referred to as beingin particle form. Although many types of phosphate salts are known, itis preferred to use diammonium hydrogen phosphate (NH₄)₂HPO₄ (DAHP) orammonium dihydrogen phosphate (NH₄)H_(z)PO₄(ADHP). Both ADHP and DAHPmeet the preferred criteria that the precursors decompose in thepresence of one another or react with one another before melting of suchprecursor. Exemplary metal compounds are Fe₂O₃, Fe₃O₄, V₂O₅, VO₂, LiVO₃,NH₄VO₃, Mg(OH)₂, Cao, MgO, Ca(OH)₂, MnO₂, Mn₂O₃, Mn₃(PO₄)₂, CuO, SnO,SnO₂, TiO₂, Ti₂O₃, Cr₂O₃, PbO₂, PbO, Ba(OH)₂, BaO, Cd(OH)₂. In addition,some starting materials serve as both the source of metal ion andphosphate, such as FePO₄, Fe₃(PO₄)₂, Zn₃(PO₄)₂, and Mg₃(PO₄)₂. Stillothers contain both lithium ion and phosphate such as Li₃PO₄ andLiH₂PO₄. Other exemplary precursors are H₃PO₄ (phosphoric acid); andP₂O₅(P₄O₁₀) phosphoric oxide; and HPO₃ meta phosphoric acid, which is adecomposition product of P₂O₅. If it is desired to replace any of theoxygen with a halogen, such as fluorine, the starting materials furtherinclude a fluorine compound such as LiF. If it is desired to replace anyof the phosphorous with silicon, then the starting materials furtherinclude silicon oxide (SiO₂). Similarly, ammonium sulfate in thestarting materials is useable to replace phosphorus with sulfur.

The starting materials are available from a number of sources. Thefollowing are typical. Vanadium pentoxide of the formula V₂O₅ isobtainable from any number of suppliers including Kerr McGee, JohnsonMatthey, or Alpha Products of Davers, Mass. Vanadium pentoxide has a CASnumber of 1314-62-1. Iron oxide Fe₃O₃ is a common and very inexpensivematerial available in powder form from the same suppliers. The otherprecursor materials mentioned above are also available from well knownsuppliers, such as those listed above.

The method of the invention may also be used to react starting materialsin the presence of carbon to form a variety of other novel products,such as gamma-LiV₂O₅ and also to produce known products. Here, thecarbon functions to reduce metal ion of a starting metal compound toprovide a product containing such reduced metal ion. The method isparticularly useful to also add lithium to the resultant product, whichthus contains the metallic element ions, namely, the lithium ion and theother metal ion, thereby forming a mixed metal product. An example isthe reaction of vanadium pentoxide (V₂O₅) with lithium carbonate in thepresence of carbon to form gamma-LiV₂O₅. Here the starting metal ionV⁺⁵V⁺⁵ is reduced to V⁺⁴V⁺⁵ in the final product. A single phasegamma-LiV₂O₅ product is not known to have been directly andindependently formed before.

As described earlier, it is desirable to conduct the reaction at atemperature where the lithium compound reacts before melting. Thetemperature should be about 400° C. or greater, and desirably 450° C. orgreater, and preferably 500° C. or greater, and generally will proceedat a faster rate at higher temperatures. The various reactions involveproduction of CO or CO₂ as an effluent gas. The equilibrium at highertemperature favors CO formation. Some of the reactions are moredesirably conducted at temperatures greater than 600° C.; most desirablygreater than 650° C.; preferably 700° C. or greater; more preferably750° C. or greater. Suitable ranges for many reactions are about 700 to950° C., or about 700 to 800° C.

Generally, the higher temperature reactions produce CO effluent and thestoichiometry requires more carbon be used than the case where CO₂effluent is produced at lower temperature. This is because the reducingeffect of the C to CO₂ reaction is greater than the C to CO reaction.The C to CO₂ reaction involves an increase in carbon oxidation state of+4 (from 0 to 4) and the C to CO reaction involves an increase in carbonoxidation state of +2 (from ground state zero to 2). Here, highertemperature generally refers to a range of about 650° C. to about 1000°C. and lower temperature refers to up to about 650° C. Temperatureshigher than 1200° C. are not thought to be needed.

In one aspect, the method of the invention utilizes the reducingcapabilities of carbon in a unique and controlled manner to producedesired products having structure and lithium content suitable forelectrode active materials. The method of the invention makes itpossible to produce products containing lithium, metal and oxygen in aneconomical and convenient process. The ability to lithiate precursors,and change the oxidation state of a metal without causing abstraction ofoxygen from a precursor is heretofore unexpected. These advantages areat least in part achieved by the reductant, carbon, having an oxidewhose free energy of formation becomes more negative as temperatureincreases. Such oxide of carbon is more stable at high temperature thanat low temperature. This feature is used to produce products having oneor more metal ions in a reduced oxidation state relative to theprecursor metal ion oxidation state. The method utilizes an effectivecombination of quantity of carbon, time and temperature to produce newproducts and to produce known products in a new way.

Referring back to the discussion of temperature, at about 700° C. boththe carbon to carbon monoxide and the carbon to carbon dioxide reactionsare occurring. At closer to 600° C. the C to CO₂ reaction is thedominant reaction. At closer to 800° C. the C to CO reaction isdominant. Since the reducing effect of the C to CO₂ reaction is greater,the result is that less carbon is needed per atomic unit of metal to bereduced. In the case of carbon to carbon monoxide, each atomic unit ofcarbon is oxidized from ground state zero to plus 2. Thus, for eachatomic unit of metal ion (M) which is being reduced by one oxidationstate, one half atomic unit of carbon is required. In the case of thecarbon to carbon dioxide reaction, one quarter atomic unit of carbon isstoichiometrically required for each atomic unit of metal ion (M) whichis reduced by one oxidation state, because carbon goes from ground statezero to a plus 4 oxidation state. These same relationships apply foreach such metal ion being reduced and for each unit reduction inoxidation state desired.

It is preferred to heat the starting materials at a ramp rate of afraction of a degree to 10° C. per minute and preferably about 2° C. perminute. Once the desired reaction temperature is attained, the reactants(starting materials) are held at the reaction temperature for severalhours. The heating is preferably conducted under non-oxidizing or inertgas such as argon or vacuum. Advantageously, a reducing atmosphere isnot required, although it may be used if desired. After reaction, theproducts are preferably cooled from the elevated temperature to ambient(room) temperature (i.e., 10° C. to 40° C.). Desirably, the coolingoccurs at a rate similar to the earlier ramp rate, and preferably 2°C./minute cooling. Such cooling rate has been found to be adequate toachieve the desired structure of the final product. It is also possibleto quench the products at a cooling rate on the order of about 100°C./minute. In some instances, such rapid cooling (quench) may bepreferred.

The present invention resolves the capacity problem posed by widely usedcathode active material. It has been found that the capacity andcapacity retention of cells having the preferred active material of theinvention are improved over conventional materials. Optimized cellscontaining lithium-mixed metal phosphates of the invention potentiallyhave performance improved over commonly used lithium metal oxidecompounds. Advantageously, the new method of making the novellithium-mixed metal phosphate compounds of the invention is relativelyeconomical and readily adaptable to commercial production.

Objects, features, and advantages of the invention include anelectrochemical cell or battery based on lithium-mixed metal phosphates.Another object is to provide an electrode active material which combinesthe advantages of good discharge capacity and capacity retention. It isalso an object of the present invention to provide electrodes which canbe manufactured economically. Another object is to provide a method forforming electrode active material which lends itself to commercial scaleproduction for preparation of large quantities.

These and other objects, features, and advantages will become apparentfrom the following description of the preferred embodiments, claims, andaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the results of an x-ray diffraction analysis, of theLiFePO₄ prepared according to the invention using CuKα radiation,λ=1.5405 Å. Bars refer to simulated pattern from refined cellparameters, Space Group, SG=Pnma (62). The values are a=10.2883 Å(0.0020), b=5.9759 Å (0.0037), c=4.6717 Å (0.0012) 0.0072, cellvolume=287.2264 Å³ (0.0685). Density, p=3.605 g/cc, zero=0.452 (0.003).Peak at full width half maximum, PFWHM=0.21. Crystallite size from XRDdata=704 Å.

FIG. 2 is a voltage/capacity plot of LiFePO₄-containing cathode cycledwith a lithium metal anode using constant current cycling at ±0.2milliamps per square centimeter in a range of 2.5 to 4.0 volts at atemperature of about 23° C. The cathode contained 19.0 mg of the LiFePO₄active material, prepared by the method of the invention. Theelectrolyte comprised ethylene carbonate (EC) and dimethyl carbonate(DMC) in a weight ratio of 2:1 and included a 1 molar concentration ofLiPF₆ salt. The lithium-metal-phosphate containing electrode and thelithium metal counter electrode are maintained spaced apart by a glassfiber separator which is interpenetrated by the solvent and the salt.

FIG. 3 shows multiple constant current cycling of LiFePO₄ activematerial cycled with a lithium metal anode using the electrolyte asdescribed in connection with FIG. 2 and cycled, charge and discharge at±0.2 milliamps per square centimeter, 2.5 to 4.0 volts at two differenttemperature conditions, 23° C. and 60° C. FIG. 3 shows the excellentrechargeability of the lithium iron phosphate/lithium metal cell, andalso shows the excellent cycling and specific capacity (mAh/g) of theactive material.

FIG. 4 shows the results of an x-ray diffraction analysis, of theLiFe_(0.9)Mg_(0.1)PO₄ prepared according to the invention, using CuKαradiation, λ=1.5405 Å. Bars refer to simulated pattern from refined cellparameters SG=Pnma (62). The values are a=10.2688 Å (0.0069), b=5.9709 Å(0.0072), c=4.6762 Å (0.0054), cell volume=286.7208 Å (0.04294), p=3.617g/cc, zero=0.702 (0.003), PFWHM=0.01, and crystallite=950 Å.

FIG. 5 is a voltage/capacity plot of LiFe_(0.9)Mg_(0.1)PO₄-containingcathode cycled with a lithium metal anode using constant current cyclingat ±0.2 milliamps per square centimeter in a range of 2.5 to 4.0 volts.Other conditions are as described earlier with respect to FIG. 2. Thecathode contained 18.9 mg of the LiFe_(0.9)Mg_(0.1)PO₄ active materialprepared by the method of the invention.

FIG. 6 shows multiple constant current cycling of LiFe_(0.9)Mg_(0.1)PO₄cycled with a lithium metal anode using the electrolyte as described inconnection with FIG. 2 and cycled, charge and discharge at ±0.2milliamps per square centimeter, 2.5 to 4.0 volts at two differenttemperature conditions, 23° C. and 60° C. FIG. 6 shows the excellentrechargeability of the lithium-metal-phosphate/lithium metal cell, andalso shows the excellent cycling and capacity of the cell.

FIG. 7 is a voltage/capacity plot of LiFe_(0.8)Mg_(0.2)PO₄-containingcathode cycled with a lithium metal anode using constant current cyclingat ±0.2 milliamps per square centimeter in a range of 2.5 to 4.0 voltsat 23° C. Other conditions are as described earlier with respect to FIG.2. The cathode contained 16 mg of the LiFe_(0.8)Mg_(0.2)PO₄ activematerial prepared by the method of the invention.

FIG. 8 shows the results of an x-ray diffraction analysis, of theLiFe_(0.9)Ca_(0.1)PO₄ prepared according to the invention, using CuKαradiation, λ=1.5405 Å. Bars refer to simulated pattern from refined cellparameters SG=Pnma (62). The values are a=10.3240 Å (0.0045), b=6.0042 Å(0.0031), c=4.6887 Å (0.0020), cell volume=290.6370 Å (0.1807),zero=0.702 (0.003), p=3.62 g/cc, PFWHM=0.18, and crystallite=680 Å.

FIG. 9 is a voltage/capacity plot of LiFe_(0.8)Ca_(0.2)PO₄-containingcathode cycled with a lithium metal anode using constant current cyclingat ±0.2 milliamps per square centimeter in a range of 2.5 to 4.0 voltsat 23°. Other conditions are as described earlier with respect to FIG.2. The cathode contained 18.5 mg of the LiFe_(0.8)Ca_(0.2)PO₄ activematerial prepared by the method of the invention.

FIG. 10 is a voltage/capacity plot of LiFe_(0.8)Zn_(0.2)PO₄-containingcathode cycled with a lithium metal anode using constant current cyclingat ±0.2 milliamps per square centimeter in a range of 2.5 to 4.0 voltsat 23° C. Other conditions are as described earlier with respect to FIG.2. The cathode contained 18.9 mg of the LiFe_(0.8)Zn_(0.2)PO₄ activematerial prepared by the method of the invention.

FIG. 11 shows the results of an x-ray diffraction analysis of thegamma-Li_(x)V₂O₅(x=1, gamma LiV₂O₅) prepared according to the inventionusing CuKα radiation λ=1.5405 Å. The values are a=9.687 Å (1), b=3.603 Å(2), and c=10.677 Å (3); phase type is gamma-Li_(x)V₂O₅(x=1); symmetryis orthorhombic; and space group is Pnma.

FIG. 12 is a voltage/capacity plot of gamma-LiV₂O₅-containing cathodecycled with a lithium metal anode using constant current cycling at ±0.2milliamps per square centimeter in a range of 2.5 to 3.8 volts at 23° C.Other conditions are as described earlier with respect to FIG. 2. Thecathode contained 21 mg of the gamma-LiV₂O₅ active material prepared bythe method of the invention.

FIG. 13 is a two-part graph based on multiple constant current cyclingof gamma-LiV₂O₅ cycled with a lithium metal anode using the electrolyteas described in connection with FIG. 2 and cycled, charge and dischargeat ±0.2 milliamps per square centimeter, 2.5 to 3.8 volts. In thetwo-part graph, FIG. 13 shows the excellent rechargeability of thelithium-metal-oxide/lithium metal cell. FIG. 13 shows the excellentcycling and capacity of the cell.

FIG. 14 shows the results of an x-ray diffraction analysis of theLi₃V₂(PO₄)₃ prepared according to the invention. The analysis is basedon CuKα radiation, λ=1.5405 Å. The values are a=12.184 Å (2), b=8.679 Å(2), c=8.627 Å (3), and β=90.457° (4).

FIG. 15 shows the results of an x-ray diffraction analysis ofLi₃V₂(PO₄)₃ prepared according to a method described in U.S. Pat. No.5,871,866. The analysis is based on CuKα radiation, λ=1.5405 Å. Thevalues are a=12.155 Å (2), b=8.711 Å (2), c=8.645 Å (3); the angle betais 90.175 (6); symmetry is Monoclinic; and space group is P2₁/n.

FIG. 16 is an EVS (Electrochemical Voltage Spectroscopy)voltage/capacity profile for a cell with cathode material formed by thecarbothermal reduction method of the invention. The cathode material is13.8 mg of Li₃V₂(PO₄)₃. The cell includes a lithium metal counterelectrode in an electrolyte comprising ethylene carbonate (EC) anddimethyl carbonate (DMC) in a weight ratio of 2:1 and including a 1molar concentration of LiPF₆ salt. The lithium-metal-phosphatecontaining electrode and the lithium metal counter electrode aremaintained spaced apart by a fiberglass separator which isinterpenetrated by the solvent and the salt. The conditions are ±10 mVsteps, between about 3.0 and 4.2 volts, and the critical limitingcurrent density is less than or equal to 0.1 mA/cm².

FIG. 17 is an EVS differential capacity versus voltage plot for the cellas described in connection with FIG. 16.

FIG. 18 shows multiple constant current cycling of LiFe_(0.8)Mg_(0.2)PO₄cycled with a lithium metal anode using the electrolyte as described inconnection with FIG. 2 and cycled, charge and discharge at ±0.2milliamps per square centimeter, 2.5 to 4.0 volts at two differenttemperature conditions, 23° C. and 60° C. FIG. 18 shows the excellentrechargeability of the lithium-metal-phosphate/lithium metal cell, andalso shows the excellent cycling and capacity of the cell.

FIG. 19 is a graph of potential over time for the first four completecycles of the LiMg_(0.1)Fe_(0.9)PO₄/MCMB graphite cell of the invention.

FIG. 20 is a two-part graph based on multiple constant current cyclingof LiFe_(0.9)Mg_(0.1)PO₄ cycled with an MCMB graphite anode using theelectrolyte as described in connection with FIG. 2 and cycled, chargeand discharge at ±0.2 milliamps per square centimeter, 2.5 to 3.6 volts,23° C. and based on a C/10 (10 hour) rate. In the two-part graph, FIG.20 shows the excellent rechargeability of thelithium-metal-phosphate/graphite cell. FIG. 20 shows the excellentcycling and capacity of the cell.

FIG. 21 is a graph of potential over time for the first three completecycles of the gamma-LiV₂O₅/MCMB graphite cell of the invention.

FIG. 22 is a diagrammatic representation of a typical laminatedlithium-ion battery cell structure.

FIG. 23 is a diagrammatic representation of a typical multi-cell batterycell structure.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention provides lithium-mixed metal-phosphates, which areusable as electrode active materials, for lithium (Li⁺) ion removal andinsertion. Upon extraction of the lithium ions from thelithium-mixed-metal-phosphates, significant capacity is achieved. In oneaspect of the invention, electrochemical energy is provided whencombined with a suitable counter electrode by extraction of a quantity xof lithium from lithium-mixed-metal-phosphatesLi_(a-x)MI_(b)MII_(c)(PO₄)_(d). When a quantity x of lithium is removedper formula unit of the lithium-mixed-metal phosphate, metal MI isoxidized. In another aspect, metal MII is also oxidized. Therefore, atleast one of MI and MII is oxidizable from its initial condition in thephosphate compound as Li is removed. Consider the following whichillustrate the mixed metal compounds of the invention:LiFe_(1-y)Sn_(y)PO₄, has two oxidizable elements, Fe and Sn; incontrast, LiFe_(1-y)Mg_(y)PO₄ has one oxidizable metal, the metal Fe.

In another aspect, the invention provides a lithium ion battery whichcomprises an electrolyte; a negative electrode having an insertionactive material; and a positive electrode comprising alithium-mixed-metal-phosphate active material characterized by anability to release lithium ions for insertion into the negativeelectrode active material. The lithium-mixed-metal-phosphate isdesirably represented by the nominal general formulaLi_(a)MI_(b)MII_(c)(PO₄)_(d). Although the metals MI and MII may be thesame, it is preferred that the metals MI and MII are different.Desirably, in the phosphate compound MI is a metal selected from thegroup: Fe, Co, Ni, Mn, Cu, V, Sn, Ti, Cr and mixtures thereof, and MI ismost desirably a transition metal or mixture thereof selected from saidgroup. Most preferably, MI has a +2 valence or oxidation state.

In another aspect, MII is selected from Mg, Ca, Zn, Sr, Pb, Cd, Sn, Ba,Be, and mixtures thereof. Most preferably, MII has a +2 valence oroxidation state. The lithium-mixed-metal-phosphate is preferably acompound represented by the nominal general formulaLi_(a-x)MI_(b)MII_(c)(PO₄)_(d), signifying the preferred composition andits capability to release x lithium. Accordingly, during cycling, chargeand discharge, the value of x varies as x greater than or equal to 0 andless than or equal to a. The present invention resolves a capacityproblem posed by conventional cathode active materials. Such problemswith conventional active materials are described by Tarascon in U.S.Pat. No. 5,425,932, using LiMn₂O₄ as an example. Similar problems areobserved with LiCoO₂, LiNiO₂, and many, if not all, lithium metalchalcogenide materials. The present invention demonstrates thatsignificant capacity of the cathode active material is utilizable andmaintained.

A preferred novel procedure for forming thelithium-mixed-metal-phosphate Li_(a)MI_(b)MII_(c)(PO₄)_(d) compoundactive material will now be described. In addition, the preferred novelprocedure is also applicable to formation of other lithium metalcompounds, and will be described as such. The basic procedure will bedescribed with reference to exemplary starting materials but is notlimited thereby. The basic process comprises conducting a reactionbetween a lithium compound, preferably lithium carbonate (Li₂CO₃), metalcompound(s), for example, vanadium pentoxide (V₂O₅), iron oxide (Fe₂O₃),and/or manganese hydroxide, and a phosphoric acid derivative, preferablythe phosphoric acid ammonium salt, diammonium hydrogen phosphate,(NH₄)₂H(PO₄). Each of the precursor starting materials are availablefrom a number of chemical outfits including Aldrich Chemical Company andFluka. Using the method described herein, LiFePO₄ andLiFe_(0.9)Mg_(0.1)PO₄, Li₃V₂(PO₄)₃ were prepared with approximately astoichiometric amount of Li₂CO₃, the respective metal compound, and(NH₄)₂HPO₄. Carbon powder was included with these precursor materials.The precursor materials were initially intimately mixed and dry groundfor about 30 minutes. The intimately mixed compounds were then pressedinto pellets. Reaction was conducted by heating in an oven at apreferred ramped heating rate to an elevated temperature, and held atsuch elevated temperature for several hours to complete formation of thereaction product. The entire reaction was conducted in a non-oxidizingatmosphere, under flowing pure argon gas. The flow rate will depend uponthe size of the oven and the quantity needed to maintain the atmosphere.The oven was permitted to cool down at the end of the reaction period,where cooling occurred at a desired rate under argon. Exemplary andpreferred ramp rates, elevated reaction temperatures and reaction timesare described herein. In one aspect, a ramp rate of 2°/minute to anelevated temperature in a range of 750° C. to 800° C. was suitable alongwith a dwell (reaction time) of 8 hours. Refer to Reactions 1, 2, 3 and4 herein. In another variation per Reaction 5, a reaction temperature of600° C. was used along with a dwell time of about one hour. In stillanother variation, as per Reaction 6, a two-stage heating was conducted,first to a temperature of 300° C. and then to a temperature of 850°.

The general aspects of the above synthesis route are applicable to avariety of starting materials. Lithium-containing compounds include Li₂O(lithium oxide), LiH₂PO₄(lithium hydrogen phosphate), Li₂C₂O₄(lithiumoxalate), LiOH (lithium hydroxide), LiOH.H₂O (lithium hydroxidemonohydride), and LiHCO₃ (lithium hydrogen carbonate). The metalcompounds(s) are reduced in the presence of the reducing agent, carbon.The same considerations apply to other lithium-metal- andphosphate-containing precursors. The thermodynamic considerations suchas ease of reduction, of the selected precursors, the reaction kinetics,and the melting point of the salts will cause adjustment in the generalprocedure, such as, amount of carbon reducing agent, and the temperatureof reaction.

FIGS. 1 through 21 which will be described more particularly below showcharacterization data and capacity in actual use for the cathodematerials (positive electrodes) of the invention. Some tests wereconducted in a cell comprising a lithium metal counter electrode(negative electrode) and other tests were conducted in cells having acarbonaceous counter electrode. All of the cells had an EC:DMC-LiPF₆electrolyte.

Typical cell configurations will now be described with reference toFIGS. 22 and 23; and such battery or cell utilizes the novel activematerial of the invention. Note that the preferred cell arrangementdescribed here is illustrative and the invention is not limited thereby.Experiments are often performed, based on full and half cellarrangements, as per the following description. For test purposes, testcells are often fabricated using lithium metal electrodes. When formingcells for use as batteries, it is preferred to use an insertion positiveelectrode as per the invention and a graphitic carbon negativeelectrode.

A typical laminated battery cell structure 10 is depicted in FIG. 22. Itcomprises a negative electrode side 12, a positive electrode side 14,and an electrolyte/separator 16 there between. Negative electrode side12 includes current collector 18, and positive electrode side 14includes current collector 22. A copper collector foil 18, preferably inthe form of an open mesh grid, upon which is laid a negative electrodemembrane 20 comprising an insertion material such as carbon or graphiteor low-voltage lithium insertion compound, dispersed in a polymericbinder matrix. An electrolyte/separator film 16 membrane is preferably aplasticized copolymer. This electrolyte/separator preferably comprises apolymeric separator and a suitable electrolyte for ion transport. Theelectrolyte/separator is positioned upon the electrode element and iscovered with a positive electrode membrane 24 comprising a compositionof a finely divided lithium insertion compound in a polymeric bindermatrix. An aluminum collector foil or grid 22 completes the assembly.Protective bagging material 40 covers the cell and prevents infiltrationof air and moisture.

In another embodiment, a multi-cell battery configuration as per FIG. 23is prepared with copper current collector 51, negative electrode 53,electrolyte/separator 55, positive electrode 57, and aluminum currentcollector 59. Tabs 52 and 58 of the current collector elements formrespective terminals for the battery structure. As used herein, theterms “cell” and “battery” refer to an individual cell comprisinganode/electrolyte/cathode and also refer to a multi-cell arrangement ina stack.

The relative weight proportions of the components of the positiveelectrode are generally: 50–90% by weight active material; 5–30% carbonblack as the electric conductive diluent; and 3–20% binder chosen tohold all particulate materials in contact with one another withoutdegrading ionic conductivity. Stated ranges are not critical, and theamount of active material in an electrode may range from 25–95 weightpercent. The negative electrode comprises about 50–95% by weight of apreferred graphite, with the balance constituted by the binder. Atypical electrolyte separator film comprises approximately two partspolymer for every one part of a preferred fumed silica. The conductivesolvent comprises any number of suitable solvents and salts. Desirablesolvents and salts are described in U.S. Pat. Nos. 5,643,695 and5,418,091. One example is a mixture of EC:DMC:LiPF₆ in a weight ratio ofabout 60:30:10.

Solvents are selected to be used individually or in mixtures, andinclude dimethyl carbonate (DMC), diethylcarbonate (DEC),dipropylcarbonate (DPC), ethylmethylcarbonate (EMC), ethylene carbonate(EC), propylene carbonate (PC), butylene carbonate, lactones, esters,glymes, sulfoxides, sulfolanes, etc. The preferred solvents are EC/DMC,EC/DEC, EC/DPC and EC/EMC. The salt content ranges from 5% to 65% byweight, preferably from 8% to 35% by weight.

Those skilled in the art will understand that any number of methods areused to form films from the casting solution using conventional meterbar or doctor blade apparatus. It is usually sufficient to air-dry thefilms at moderate temperature to yield self-supporting films ofcopolymer composition. Lamination of assembled cell structures isaccomplished by conventional means by pressing between metal plates at atemperature of about 120–160° C. Subsequent to lamination, the batterycell material may be stored either with the retained plasticizer or as adry sheet after extraction of the plasticizer with a selectivelow-boiling point solvent. The plasticizer extraction solvent is notcritical, and methanol or ether are often used.

Separator membrane element 16 is generally polymeric and prepared from acomposition comprising a copolymer. A preferred composition is the 75 to92% vinylidene fluoride with 8 to 25% hexafluoropropylene copolymer(available commercially from Atochem North America as Kynar FLEX) and anorganic solvent plasticizer. Such a copolymer composition is alsopreferred for the preparation of the electrode membrane elements, sincesubsequent laminate interface compatibility is ensured. The plasticizingsolvent may be one of the various organic compounds commonly used assolvents for electrolyte salts, e.g., propylene carbonate or ethylenecarbonate, as well as mixtures of these compounds. Higher-boilingplasticizer compounds such as dibutyl phthalate, dimethyl phthalate,diethyl phthalate, and tris butoxyethyl phosphate are particularlysuitable. Inorganic filler adjuncts, such as fumed alumina or silanizedfumed silica, may be used to enhance the physical strength and meltviscosity of a separator membrane and, in some compositions, to increasethe subsequent level of electrolyte solution absorption.

In the construction of a lithium-ion battery, a current collector layerof aluminum foil or grid is overlaid with a positive electrode film, ormembrane, separately prepared as a coated layer of a dispersion ofinsertion electrode composition. This is typically an insertion compoundsuch as LiMn₂O₄(LMO), LiCoO₂, or LiNiO₂, powder in a copolymer matrixsolution, which is dried to form the positive electrode. Anelectrolyte/separator membrane is formed as a dried coating of acomposition comprising a solution containing VdF:HFP copolymer and aplasticizer solvent is then overlaid on the positive electrode film. Anegative electrode membrane formed as a dried coating of a powderedcarbon or other negative electrode material dispersion in a VdF:HFPcopolymer matrix solution is similarly overlaid on the separatormembrane layer. A copper current collector foil or grid is laid upon thenegative electrode layer to complete the cell assembly. Therefore, theVdF:HFP copolymer composition is used as a binder in all of the majorcell components, positive electrode film, negative electrode film, andelectrolyte/separator membrane. The assembled components are then heatedunder pressure to achieve heat-fusion bonding between the plasticizedcopolymer matrix electrode and electrolyte components, and to thecollector grids, to thereby form an effective laminate of cell elements.This produces an essentially unitary and flexible battery cellstructure.

Examples of forming cells containing metallic lithium anode, insertionelectrodes, solid electrolytes and liquid electrolytes can be found inU.S. Pat. Nos. 4,668,595; 4,830,939; 4,935,317; 4,990,413; 4,792,504;5,037,712; 5,262,253; 5,300,373; 5,435,054; 5,463,179; 5,399,447;5,482,795 and 5,411,820; each of which is incorporated herein byreference in its entirety. Note that the older generation of cellscontained organic polymeric and inorganic electrolyte matrix materials,with the polymeric being most preferred. The polyethylene oxide of U.S.Pat. No. 5,411,820 is an example. More modern examples are the VdF:HFPpolymeric matrix. Examples of casting, lamination and formation of cellsusing VdF:HFP are as described in U.S. Pat. Nos. 5,418,091; 5,460,904;5,456,000; and 5,540,741; assigned to Bell Communications Research, eachof which is incorporated herein by reference in its entirety.

As described earlier, the electrochemical cell operated as per theinvention, may be prepared in a variety of ways. In one embodiment, thenegative electrode may be metallic lithium. In more desirableembodiments, the negative electrode is an insertion active material,such as, metal oxides and graphite. When a metal oxide active materialis used, the components of the electrode are the metal oxide,electrically conductive carbon, and binder, in proportions similar tothat described above for the positive electrode. In a preferredembodiment, the negative electrode active material is graphiteparticles. For test purposes, test cells are often fabricated usinglithium metal electrodes. When forming cells for use as batteries, it ispreferred to use an insertion metal oxide positive electrode and agraphitic carbon negative electrode. Various methods for fabricatingelectrochemical cells and batteries and for forming electrode componentsare described herein. The invention is not, however, limited by anyparticular fabrication method.

Formation of Active Materials

EXAMPLE I

Reaction 1(a). LiFePO₄ formed from FePO₄FePO₄+0.5 Li₂CO₃+0.5 C→LiFePO₄+0.5 CO₂₊0.5 CO

-   -   (a) Pre-mix reactants in the following proportions using ball        mill. Thus,

1 mol FePO₄ 150.82 g 0.5 mol Li₂CO₃ 36.95 g 0.5 mol carbon 6.0 g

-   -    (but use 100% excess carbon→12.00 g)    -   (b) Pelletize powder mixture    -   (c) Heat pellet to 750° C. at a rate of 2°/minute in flowing        inert atmosphere (e.g. argon). Dwell for 8 hours at 750° C.        under argon.    -   (d) Cool to room temperature at 2°/minute under argon.    -   (e) Powderize pellet.    -   Note that at 750° C. this is predominantly a CO reaction. This        reaction is able to be conducted at a temperature in a range of        about 700° C. to about 950° C. in argon as shown, and also under        other inert atmospheres such as nitrogen or vacuum.

EXAMPLE II

Reaction 1(b). LiFePO₄ formed from Fe₂O₃0.5 Fe₂O₃+0.5 Li₂CO₃+(NH₄)₂HPO₄+0.5 C→LiFePO₄+0.5 CO₂+2 NH₃+3/2 H₂O+0.5CO

-   -   (a) Premix powders in the following proportions

0.5 mol Fe₂O₃ 79.85 g 0.5 mol Li₂CO₃ 36.95 g 1 mol (NH₄)₂HPO₄ 132.06 g0.5 mol carbon 6.00 g

-   -    (use 100% excess carbon→12.00 g)    -   (b) Pelletize powder mixture    -   (c) Heat pellet to 750° C. at a rate of 2°/minute in flowing        inert atmosphere (e.g. argon). Dwell for 8 hours at 750° C.        under argon.    -   (d) Cool to room temperature at 2°/minute under argon.    -   (e) Powderize

EXAMPLE III

Reaction 1(c). LiFePO₄—from Fe₃(PO₄)₂

Two steps:

Part I. Carbothermal Preparation of Fe₃(PO₄)₂3/2 Fe₂O₃+2(NH₄)₂HPO₄+3/2 C→Fe₃(PO₄)₂+3/2 CO+4NH₃+5/2 H₂O

-   -   (a) Premix reactants in the following proportions

3/2 mol Fe₂O₃ 239.54 g 2 mol (NH₄)₂HPO₄ 264.12 g 3/2 mol carbon 18.00 g

-   -   (use 100% excess carbon→36.00 g)    -   (b) Pelletize powder mixture    -   (c) Heat pellet to 800° C. at a rate of 2°/minute in flowing        inert atmosphere (e.g. argon). Dwell for 8 hours at 750° C.        under argon.    -   (d) Cool to room temperature at 2° C./minute under argon.    -   (e) Powderize pellet.

Part II. Preparation of LiFePO₄ from the Fe₃(PO₄)₂ of Part I.Li₃PO₄+Fe(PO₄)₂→3 LiFePO₄

-   -   (a) Premix reactants in the following proportions

1 mol Li₃PO₄ 115.79 g 1 mol Fe₃(PO₄)₂ 357.48 g

-   -   (b) Pelletize powder mixture    -   (c) Heat pellet to 750° C. at a rate of 2°/minute in flowing        inert atmosphere (e.g. argon). Dwell for 8 hours at 750° C.        under argon.    -   (d) Cool to room temperature at 2° C./minute under argon.    -   (e) Powderize pellet.

EXAMPLE IV

Reaction 2(a). LiFe_(0.9)Mg_(0.1)PO₄(LiFe_(1-y)Mg_(y)PO₄) formed fromFePO₄0.5 Li₂CO₃+0.9 FePO₄+0.1Mg(OH)₂+0.1(NH₄)₂HPO₄+0.45C→LiFe_(0.9)Mg_(0.1)PO₄+0.5CO₂+0.45CO+0.2NH₃+0.25H₂O

-   -   (a) Pre-mix reactants in the following proportions        -   0.50 mol Li₂CO₃=36.95 g        -   0.90 mol FePO₄=135.74 g        -   0.10 mol Mg(OH)₂=5.83 g        -   0.10 mol (NH₄)₂HPO₄=1.32 g        -   0.45 mol carbon=5.40 g    -   (use 100% excess carbon→10.80g)    -   (b) Pelletize powder mixture    -   (c) Heat to 750° C. at a rate of 2°/minute in argon. Hold for 8        hours dwell at 750° C. in argon    -   (d) Cool at a rate of 2°/minute    -   (e) Powderize pellet.

EXAMPLE V

Reaction 2(b). LiFe_(0.9)Mg_(0.1)PO₄(LiFe_(1-y)Mg_(y)PO₄) formed fromFe₂O₃0.50 Li₂CO₃+0.45 Fe₂O₃+0.10 Mg(OH)₂+(NH₄)₂HPO₄+0.45C→LiFe_(0.9)Mg_(0.1)PO₄+0.5 CO₂+0.45 CO+2 NH₃+1.6H₂O

-   -   (a) Pre-mix reactants in following ratio        -   0.50 mol Li₂CO₃=36.95 g        -   0.45 mol Fe₂O₃=71.86 g        -   0.10 mol Mg(OH)₂=5.83 g        -   1.00 mol (NH₄)₂HPO₄=132.06 g        -   0.45 mol carbon=5.40 g    -   (use 100% excess carbon→10.80 g)    -   (b) Pelletize powder mixture    -   (c) Heat to 750° C. at a rate of 2°/minute in argon. Hold for 8        hours dwell at 750° C. in argon    -   (d) Cool at a rate of 2°/minute    -   (e) Powderize pellet.

EXAMPLE VI

Reaction 2(c). LiFe_(0.9)Mg_(0.1)PO₄(LiFe_(1-y)Mg_(y)PO₄) formed fromLiH₂PO₄1.0 LiH₂PO₄+0.45 Fe₂O₃+0.10 Mg(OH)₂+0.45C→LiFe_(0.9)Mg_(0.1)PO₄+0.45CO+1.1 H₂O

-   -   (a) Pre-mix reactants in the following proportions        -   1.00 mol LiH₂PO₄=103.93 g        -   0.45 mol Fe₂O₃=71.86 g        -   0.10 mol Mg(OH)₂=5.83 g        -   0.45 mol carbon=5.40 g    -   (use 100% excess carbon→10.80 g)    -   (b) Pelletize powder mixture    -   (c) Heat to 750° C. at a rate of 2°/minute in argon. Hold for 8        hours dwell at 750° C. in argon    -   (d) Cool at a rate of 2°/minute    -   (e) Powderize pellet.

EXAMPLE VII

Reaction 3. Formation of LiFe_(0.9)Ca_(0.1)PO₄ (LiFe_(1-y)Ca_(y)PO₄)from Fe₂O₃0.50 Li₂CO₃+0.45 Fe₂O₃+0.1Ca(OH)₂+(NH₄)₂HPO₄+0.45C→LiFe_(0.9)Ca_(0.1)PO₄+0.5 CO₂+0.45 CO+2 NH₃+1.6H₂O

-   -   (a) Pre-mix reactants in the following proportions        -   0.50 mol Li₂CO₃=36.95 g        -   0.45 mol Fe₂O₃=71.86 g        -   0.10 mol Ca(OH)₂=7.41 g        -   1.00 mol (NH₄)₂HPO₄=132.06 g        -   0.45 mol carbon=5.40 g    -   (100% excess carbon→10.80 g)    -   (b) Pelletize powder mixture    -   (c) Heat to 750° C. at a rate of 2°/minute in argon. Hold for 8        hours dwell at 750° C. in argon    -   (d) Cool at a rate of 2°/minute    -   (e) Powderize pellet.

EXAMPLE VIII

Reaction 4. Formation of LiFe_(0.9)Zn_(0.1)PO₄ (LiFe_(1-y)Zn_(y)PO₄)from Fe₂O₃.0.50 Li₂CO₃+0.45 Fe₂O₃+0.033 Zn₃(PO₄)₂+0.933(NH₄)₂HPO₄+0.45C→LiFe_(0.9)Zn_(0.1)PO₄+0.50 CO₂+0.45 CO+1.866 NH₃+1.2H₂O

-   -   Pre-mix reactants in the following proportions        -   0.50 mol Li₂CO₃=36.95 g        -   0.45 mol Fe₂O₃=71.86 g        -   0.033 mol Zn₃(PO₄)₂=12.74 g        -   0.933 mol (NH₄)₂HPO₄=123.21 g        -   0.45 mol carbon=5.40 g    -   (100% excess carbon→10.80 g)    -   (b) Pelletize powder mixture    -   (c) Heat to 750° C. at a rate of 2°/minute in argon. Hold for 8        hours dwell at 750° C. in argon    -   (d) Cool at a rate of 2°/minute    -   (e) Powderize pellet.

EXAMPLE IX

Reaction 5. Formation of gamma-LiV₂O₅(γ)V₂O₅+0.5 Li₂CO₃+0.25 C→LiV₂O₅+3/4 CO₂

-   -   (a) Pre-mix alpha V₂O₅, Li₂CO₃ and Shiwinigan Black (carbon)        using bail mix with suitable media. Use a 25% weight excess of        carbon over the reaction amounts above. For example, according        to reaction above:        -   Need:

1 mol V₂O₅ 181.88 g 0.5 mol Li₂CO₃ 36.95 g 0.25 mol carbon 3.00 g

-   -   -   (but use 25% excess carbon→3.75 g)

    -   (b) Pelletize powder mixture

    -   (c) Heat pellet to 600° C. in flowing argon (or other inert        atmosphere) at a heat rate of approximately 2°/minute. Hold at        600° C. for about 60 minutes.

    -   (d) Allow to cool to room temperature in argon at cooling rate        of about 2°/minute.

    -   (e) Powderize pellet using mortar and pestle

This reaction is able to be conducted at a temperature in a range ofabout 400° C. to about 650° C. in argon as shown, and also under otherinert atmospheres such as nitrogen or vacuum. This reaction at thistemperature range is primarily C→CO₂. Note that the reaction C→COprimarily occurs at a temperature over about 650° C. (HT, hightemperature); and the reaction C→CO₂ primarily occurs at a temperatureof under about 650° C. (LT, low temperature). The reference to about650° C. is approximate and the designation “primarily” refers to thepredominant reaction thermodynamically favored although the alternatereaction may occur to some extent.

EXAMPLE X

Reaction 6. Formation of Li₃V₂(PO₄)₃V₂O₅+3/2 Li₂CO₃+3(NH₄)₂HPO₄+C→Li₃V₂(PO₄)₃+2 CO+3/2 CO₂+6 NH₃+9/2 H₂O

-   -   (a) Pre-mix reactants above using ball mill with suitable media.        Use a 25% weight excess of carbon. Thus,

1 mol V₂O₅ 181.88 g 3/2 mol Li₂CO₃ 110.84 g 3 mol (NH₄)₂HPO₄ 396.18 g 1mol carbon 12.01 g

-   -   -   (but use 25% excess carbon→15.01 g)

    -   (b) Pelletize powder mixture

    -   (c) Heat pellet at 2°/minute to 300° C. to remove CO₂ (from        Li₂CO₃) and to remove NH₃, H₂O. Heat in an inert atmosphere        (e.g. argon). Cool to room temperature.

    -   (d) Powderize and repelletize

    -   (e) Heat pellet in inert atmosphere at a rate of 2° C./minute to        850° C. Dwell for 8 hours at 850° C.

    -   (f) Cool to room temperature at a rate of 2°/minute in argon.

    -   (e) Powderize

This reaction is able to be conducted at a temperature in a range ofabout 700° C. to about 950° C. in argon as shown, and also under otherinert atmospheres such as nitrogen or vacuum. A reaction temperaturegreater than about 670° C. ensures C→CO reaction is primarily carriedout.

Characterization of Active Materials and Formation and Testing of Cells

Referring to FIG. 1, the final product LiFePO₄, prepared from Fe₂O₃metal compound per Reaction 1(b), appeared brown/black in color. Thisolivine material product included carbon that remained after reaction.Its CuKα x-ray diffraction pattern contained all of the peaks expectedfor this material as shown in FIG. 1. The pattern evident in FIG. 1 isconsistent with the single phase olivine phosphate, LiFePO₄. This isevidenced by the position of the peaks in terms of the scattering angle2θ (theta), x axis. The x-ray pattern showed no peaks due to thepresence of precursor oxides indicating that the solid state reaction isessentially entirely completed. Here the space group SG= pnma (62) andthe lattice parameters from XRD refinement are consistent with theolivine structure. The values are a=10.2883 Å (0.0020), b=5.9759(0.0037), c=4.6717 Å (0.0012) 0.0072, cell volume=287.2264 Å³ (0.0685).Density, p=3.605 g/cc, zero=0.452 (0.003). Peak at full width halfmaximum, PFWHM=0.21. Crystallite size from XRD data=704 Å.

The x-ray pattern demonstrates that the product of the invention wasindeed the nominal formula LiFePO₄. The term “nominal formula” refers tothe fact that the relative proportion of atomic species may varyslightly on the order of 2 percent to 5 percent, or more typically, 1percent to 3 percent, and that some portion of P may be substituted bySi, S or As; and some portion of O may be substituted by halogen,preferably F.

The LiFePO₄, prepared as described immediately above, was tested in anelectrochemical cell. The positive electrode was prepared as describedabove, using 19.0 mg of active material. The positive electrodecontained, on a weight % basis, 85% active material, 10% carbon black,and 5% EPDM. The negative electrode was metallic lithium. Theelectrolyte was a 2:1 weight ratio mixture of ethylene carbonate anddimethyl carbonate within which was dissolved 1 molar LiPF₆. The cellswere cycled between about 2.5 and about 4.0 volts with performance asshown in FIGS. 2 and 3.

FIG. 2 shows the results of the first constant current cycling at 0.2milliamps per square centimeter between about 2.5 and 4.0 volts basedupon about 19 milligrams of the LiFePO₄ active material in the cathode(positive electrode). In an as prepared, as assembled, initialcondition, the positive electrode active material is LiFePO₄. Thelithium is extracted from the LiFePO₄ during charging of the cell. Whenfully charged, about 0.72 unit of lithium had been removed per formulaunit. Consequently, the positive electrode active material correspondsto Li_(1-x)FePO₄ where x appears to be equal to about 0.72, when thecathode material is at 4.0 volts versus Li/Li⁺. The extractionrepresents approximately 123 milliamp hours per gram corresponding toabout 2.3 milliamp hours based on 19 milligrams active material. Next,the cell is discharged whereupon a quantity of lithium is re-insertedinto the LiFePO₄. The re-insertion corresponds to approximately 121milliamp hours per gram proportional to the insertion of essentially allof the lithium. The bottom of the curve corresponds to approximately 2.5volts. The total cumulative capacity demonstrated during the entireextraction-insertion cycle is 244 mAh/g.

FIG. 3 presents data obtained by multiple constant current cycling at0.2 milliamp hours per square centimeter of the LiFePO₄ versus lithiummetal counter electrode between 2.5 and 4.0 volts. Data is shown for twotemperatures, 23° C. and 60° C. FIG. 3 shows the excellentrechargeability of the LiFePO₄ cell, and also shows good cycling andcapacity of the cell. The performance shown after about 190 to 200cycles is good and shows that electrode formulation is very desirable.

Referring to FIG. 4, there is shown data for the final productLiFe_(0.9)Mg_(0.1)PO₄, prepared from the metal compounds Fe₂O₃ andMg(OH)₂→Mg(OH)₂, per Reaction 2(b). Its CuKα x-ray diffraction patterncontained all of the peaks expected for this material as shown in FIG.4. The pattern evident in FIG. 4 is consistent with the single phaseolivine phosphate compound, LiFe_(0.9)Mg_(0.1)PO₄. This is evidenced bythe position of the peaks in terms of the scattering angle 2 θ (theta),x axis. The x-ray pattern showed no peaks due to the presence ofprecursor oxides indicating that the solid state reaction is essentiallyentirely completed. Here the space group SG=Pnma (62) and the latticeparameters from XRD refinement are consistent with the olivinestructure. The values are a=10.2688 Å (0.0069), b=5.9709 Å (0.0072),c=4.6762 Å (0.0054), cell volume=286.7208 Å (0.04294), p=3.617 g/cc,zero=0.702 (0.003), PFWHM=0.01, and crystallite=950 Å.

The x-ray pattern demonstrates that the product of the invention wasindeed the nominal formula LiFe_(0.9)Mg_(0.1)PO₄ The term “nominalformula” refers to the fact that the relative proportion of atomicspecies may vary slightly on the order of 2 percent to 5 percent, ormore typically, 1 percent to 3 percent, and that some substitution of Pand O may be made while maintaining the basic olivine structure.

The LiFe_(0.9)Mg_(0.1)PO₄, prepared as described immediately above, wastested in an electrochemical cell. The positive electrode was preparedas described above, using 18.9 mg of active materials. The positiveelectrode, negative electrode and electrolyte were prepared as describedearlier and in connection with FIG. 1. The cell was between about 2.5and about 4.0 volts with performance as shown in FIGS. 4, 5 and 6.

FIG. 5 shows the results of the first constant current cycling at 0.2milliamps per square centimeter between about 2.5 and 4.0 volts basedupon about 18.9 milligrams of the LiFe_(0.9)Mg_(0.1)PO₄ active materialin the cathode (positive electrode). In an as prepared, as assembled,initial condition, the positive electrode active material isLiFe_(0.9)Mg_(0.1)PO₄. The lithium is extracted from theLiFe_(0.9)Mg_(0.1)PO₄ during charging of the cell. When fully charged,about 0.87 units of lithium have been removed per formula unit.Consequently, the positive electrode active material corresponds toLi_(1-x)Fe_(0.9)Mg_(0.1)PO₄ where x appears to be equal to about 0.87,when the cathode material is at 4.0 volts versus Li/Li⁺. The extractionrepresents approximately 150 milliamp hours per gram corresponding toabout 2.8 milliamp hours based on 18.9 milligrams active material. Next,the cell is discharged whereupon a quantity of lithium is re-insertedinto the LiFe_(0.9)Mg_(0.1)PO₄. The re-insertion corresponds toapproximately 146 milliamp hours per gram proportional to the insertionof essentially all of the lithium. The bottom of the curve correspondsto approximately 2.5 volts. The total cumulative specific capacity overthe entire cycle is 296 mAhr/g. This material has a much better cycleprofile than the LiFePO₄. FIG. 5 (LiFe_(0.9)Mg_(0.1)PO₄) shows a verywell defined and sharp peak at about 150 mAh/g. In contrast, FIG. 2(LiFePO₄) shows a very shallow slope leading to the peak at about 123mAh/g. The Fe-phosphate (FIG. 2) provides 123 mAh/g compared to itstheoretical capacity of 170 mAh/g. This ratio of 123/170, 72% isrelatively poor compared to the Fe/Mg-phosphate. The Fe/Mg-phosphate(FIG. 5) provides 150 mAh/g compared to a theoretical capacity of 160, aratio of 150/160 or 94%.

FIG. 6 presents data obtained by multiple constant current cycling at0.2 milliamp hours per square centimeter of the LiFe_(0.9)Mg_(0.1)PO₄versus lithium metal counter electrode between 2.5 and 4.0 volts. FIG. 6shows the excellent rechargeability of the Li/LiFe_(0.9)Mg_(0.1)PO₄cell, and also shows good cycling and capacity of the cell. Theperformance shown after about 150 to 160 cycles is very good and showsthat electrode formulation LiFe_(0.9)Mg_(0.1)PO₄ performed significantlybetter than the LiFePO₄. Comparing FIG. 3 (LiFePO₄) to FIG. 6(LiFe_(0.9)Mg_(0.1)PO₄) it can be seen that the Fe/Mg-phosphatemaintains its capacity over prolonged cycling, whereas the Fe-phosphatecapacity fades significantly.

FIG. 7 shows the results of the first constant current cycling at 0.2milliamps per square centimeter between about 2.5 and 4.0 volts basedupon about 16 milligrams of the LiFe_(0.8)Mg_(0.2)PO₄ active material inthe cathode (positive electrode). In an as prepared, as assembled,initial condition, the positive electrode active material isLiFe_(0.8)Mg_(0.2)PO₄. The lithium is extracted from theLiFe_(0.8)Mg_(0.2)PO₄ during charging of the cell. When fully charged,about 0.79 units of lithium have been removed per formula unit.Consequently, the positive electrode active material corresponds toLiFe_(0.8)Mg_(0.2)PO₄ where x appears to be equal to about 0.79, whenthe cathode material is at 4.0 volts versus Li/Li⁺. The extractionapproximately 135 milliamp hours per gram corresponding to about 2.2milliamp hours based on 16 milligrams active material. Next, the cell isdischarged whereupon a quantity of lithium is re-inserted into theLiFe_(0.8)Mg_(0.2)PO₄. The re-insertion corresponds to approximately 122milliamp hours per gram proportional to the insertion of essentially allof the lithium. The bottom of the curve corresponds to approximately 2.5volts. The total cumulative specific capacity over the entire cycle is262 mAhr/g.

Referring to FIG. 8, there is shown data for the final productLiFe_(0.9)Ca_(0.1)PO₄, prepared from Fe₂O₃ and Ca(OH)₂ by Reaction 3.Its CuKα x-ray diffraction pattern contained all of the peaks expectedfor this material as shown in FIG. 8. The pattern evident in FIG. 8 isconsistent with the single phase olivine phosphate compound,LiFe_(0.9)Ca_(0.1)PO₄. This is evidenced by the position of the peaks interms of the scattering angle 2θ (theta), x axis. The x-ray patternshowed no peaks due to the presence of precursor oxides indicating thatthe solid state reaction is essentially entirely completed. Here thespace group SG=Pnma (62) and the lattice parameters from XRD refinementare consistent with olivine. The values are a=10.3240 Å (0.0045),b=6.0042 Å (0.0031), c=4.6887 Å (0.0020), cell volume=290.6370 Å(0.1807), zero=0.702 (0.003), p=3.62 g/cc, PFWHM=0.18, andcrystallite=680 Å. The x-ray pattern demonstrates that the product ofthe invention was indeed the nominal formula LiFe_(0.9)Ca_(0.1)PO₄.

FIG. 9 shows the results of the first constant current cycling at 0.2milliamps per square centimeter between about 2.5 and 4.0 volts basedupon about 18.5 milligrams of the LiFe_(0.8)Ca_(0.2)PO₄ active materialin the cathode (positive electrode). In an as prepared, as assembled,initial condition, the positive electrode active material isLiFe_(0.8)Ca_(0.2)PO₄. The lithium is extracted from theLiFe_(0.8)Ca_(0.2)PO₄ during charging of the cell. When fully charged,about 0.71 units of lithium have been removed per formula unit.Consequently, the positive electrode active material corresponds toLiFe_(0.8)Ca_(0.2)PO₄ where x appears to be equal to about 0.71, whenthe cathode material is at 4.0 volts versus Li/Li⁺. The extractionrepresents approximately 123 milliamp hours per gram corresponding toabout 2.3 milliamp hours based on 18.5 milligrams active material. Next,the cell is discharged whereupon a quantity of lithium is re-insertedinto the LiFe_(0.8)Ca_(0.2)PO₄. The re-insertion corresponds toapproximately 110 milliamp hours per gram proportional to the insertionof nearly all of the lithium. The bottom of the curve corresponds toapproximately 2.5 volts. The total specific cumulative capacity over theentire cycle is 233 mAhr/g.

FIG. 10 shows the results of the first constant current cycling at 0.2milliamps per square centimeter between about 2.5 and 4.0 volts basedupon about 18.9 milligrams of the LiFe_(0.8)Zn_(0.2)PO₄ olivine activematerial in the cathode (positive electrode). In an as prepared, asassembled, initial condition, the positive electrode active material isLiFe_(0.8)Zn_(0.2)PO₄, prepared from Fe₂O₃ and Zn₃(PO₄)₂ by Reaction 4.The lithium is extracted from the LiFe_(0.8)Zn_(0.2)PO₄ during chargingof the cell. When fully charged, about 0.74 units of lithium have beenremoved per formula unit. Consequently, the positive electrode activematerial corresponds to Li_(1-x)Fe0.8Zn0.2PO4 where x appears to beequal to about 0.74, when the cathode material is at 4.0 volts versusLi/Li⁺. The extraction represents approximately 124 milliamp hours pergram corresponding to about 2.3 milliamp hours based on 18.9 milligramsactive material. Next, the cell is discharged whereupon a quantity oflithium is re-inserted into the LiFe_(0.8)Zn_(0.2)PO₄. The re-insertioncorresponds to approximately 108 milliamp hours per gram proportional tothe insertion of nearly all of the lithium. The bottom of the curvecorresponds to approximately 2.5 volts.

Referring to FIG. 11, the final product LiV₂O₅, prepared by Reaction 5,appeared black in color. Its CuKα x-ray diffraction pattern containedall of the peaks expected for this material as shown in FIG. 11. Thepattern evident in FIG. 11 is consistent with a single oxide compoundgamma-LiV₂O₅ This is evidenced by the position of the peaks in terms ofthe scattering angle 2 θ (theta), x axis. The x-ray pattern showed nopeaks due to the presence of precursor oxides indicating that the solidstate reaction is essentially entirely completed.

The x-ray pattern demonstrates that the product of the invention wasindeed the nominal formula gamma-LiV₂O₅. The term “nominal formula”refers to the fact that the relative proportion of atomic species mayvary slightly on the order of 2 percent to 5 percent, or more typically,1 percent to 3 percent.

The LiV₂O₅ prepared as described immediately above, was tested in anelectrochemical cell. The cell was prepared as described above andcycled with performance as shown in FIGS. 12 and 13.

FIG. 12 shows the results of the first constant current cycling at 0.2milliamps per square centimeter between about 2.8 and 3.8 volts basedupon about 15.0 milligrams of the LiV₂O₅ active material in the cathode(positive electrode). In an as prepared, as assembled, initialcondition, the positive electrode active material is LiV₂O₅. The lithiumis extracted from the LiV₂O₅ during charging of the cell. When fullycharged, about 0.93 unit of lithium had been removed per formula unit.Consequently, the positive electrode active material corresponds toLi_(1-x)-V₂O₅ where x appears to be equal to about 0.93, when thecathode material is at 3.8 volts versus Li/Li⁺. The extractionrepresents approximately 132 milliamp hours per gram corresponding toabout 2.0 milliamp hours based on 15.0 milligrams active material. Next,the cell is discharged whereupon a quantity of lithium is re-insertedinto the LiV₂O₅. The re-insertion corresponds to approximately 130milliamp hours per gram proportional to the insertion of essentially allof the lithium. The bottom of the curve corresponds to approximately 2.8volts.

FIG. 13 presents data obtained by multiple constant current cycling at0.4 milliamp hours per square centimeter (C/2 rate)of the LiV₂O₅ versuslithium metal counter electrode between 3.0 and 3.75 volts. Data for twotemperature conditions are shown, 23° C. and 60° C. FIG. 13 is a twopart graph with FIG. 13A showing the excellent rechargeability of theLiV₂O₅. FIG. 13B shows good cycling and capacity of the cell. Theperformance shown up to about 300 cycles is good.

Referring to FIG. 14, the final product Li₃V₂(PO₄)₃, prepared byReaction 6, appeared green/black in color. Its CuKα x-ray diffractionpattern contained all of the peaks expected for this material as shownin FIG. 14. The pattern evident in FIG. 14 is consistent with a singlephosphate compound Li₃V₂(PO₄)₃ of the monoclinic, Nasicon phase. This isevidenced by the position of the peaks in terms of the scattering angle2 θ (theta), x axis. The x-ray pattern showed no peaks due to thepresence of precursor oxides indicating that the solid state reaction isessentially entirely completed.

The x-ray pattern demonstrates that the product of the invention wasindeed the nominal formula Li₃V₂(PO₄)₃, The term “nominal formula”refers to the fact that the relative proportion of atomic species mayvary slightly on the order of 2 percent to 5 percent, or more typically,1 percent to 3 percent; and that substitution of P and O may occur.

The Li₃V₂(PO₄)₃ prepared as described immediately above, was tested inan electrochemical cell. The cell was prepared as described above, using13.8 mg of active material. The cell was prepared as described above andcycled between about 3.0 and about 4.2 volts using the EVS techniquewith performance as shown in FIGS. 16 and 17. FIG. 16 shows specificcapacity versus electrode potential against Li. FIG. 17 showsdifferential capacity versus electrode potential against Li.

A comparative method was used to form Li₃V₂(PO₄ )₃. Such method wasreaction without carbon and under H₂-reducing gas as described in U.S.Pat. No. 5,871,866. The final product, prepared as per U.S. Pat. No.5,871,866, appeared green in color. Its CuKα x-ray diffraction patterncontained all of the peaks expected for this material as shown in FIG.15. The pattern evident in FIG. 15 is consistent with a monoclinicNasicon single phase phosphate compound Li₃V₂(PO₄)₃. This is evidencedby the position of the peaks in terms of the scattering angle 2 θ(theta), x axis. The x-ray pattern showed no peaks due to the presenceof precursor oxides indicating that the solid state reaction isessentially entirely completed. Chemical analysis for lithium andvanadium by atomic absorption spectroscopy showed, on a percent byweight basis, 5.17 percent lithium and 26 percent vanadium. This isclose to the expected result of 5.11 percent lithium and 25 percentvanadium.

The chemical analysis and x-ray patterns of FIGS. 14 and 15 demonstratethat the product of FIG. 14 was the same as that of FIG. 15. The productof FIG. 14 was prepared without the undesirable H₂ atmosphere and wasprepared by the novel carbothermal solid state synthesis of theinvention.

FIG. 16 shows a voltage profile of the test cell, based on theLi₃V₂(PO₄)₃ positive electrode active material made by the process ofthe invention and as characterized in FIG. 14. It was cycled against alithium metal counter electrode. The data shown in FIG. 16 is based onthe Electrochemical Voltage Spectroscopy (EVS) technique.Electrochemical and kinetic data were recorded using the ElectrochemicalVoltage Spectroscopy (EVS) technique. Such technique is known in the artas described by J. Barker in Synth, Met 28, D217 (1989); Synth. Met. 32,43 (1989); J. Power Sources, 52, 185 (1994); and Electrochemica Acta,Vol. 40, No. 11, at 1603 (1995). FIG. 16 clearly shows and highlightsthe reversibility of the product. The positive electrode contained about13.8 milligrams of the Li₃V₂(PO₄)₃ active material. The positiveelectrode showed a performance of about 133 milliamp hours per gram onthe first discharge. In FIG. 16, the capacity in, and the capacity outare essentially the same, resulting in essentially no capacity loss.FIG. 17 is an EVS differential capacity plot based on FIG. 16. As can beseen from FIG. 17, the relatively symmetrical nature of peaks indicatesgood electrical reversibility, there are small peak separations(charge/discharge), and good correspondence between peaks above andbelow the zero axis. There are essentially no peaks that can be relatedto irreversible reactions, since all peaks above the axis (cell charge)have corresponding peaks below the axis (cell discharge), and there isessentially no separation between the peaks above and below the axis.This shows that the carbothermal method of the invention produces highquality electrode material.

FIG. 18 presents data obtained by multiple constant current cycling at0.2 milliamp hours per square centimeter of the LiFe_(0.8)Mg_(0.2)PO₄versus lithium metal counter electrode between 2.5 and 4.0 volts. FIG.18 shows the excellent rechargeability of the Li/LiFe_(0.8)Mg_(0.2)PO₄cell, and also shows good cycling and capacity of the cell. Theperformance shown after about 110 to 120 cycles at 23° C. is very goodand shows that electrode formulation LiFe_(0.8)Mg_(0.2)PO₄ performedsignificantly better than the LiFePO₄. The cell cycling test at 60° C.was started after the 23° C. test and was ongoing. Comparing FIG. 3(LiFePO₄) to FIG. 18 (LiFe_(0.8)Mg_(0.2)PO₄), it can be seen that theFe/Mg-phosphate maintains its capacity over prolonged cycling, whereasthe Fe-phosphate capacity fades significantly.

In addition to the above cell tests, the active materials of theinvention were also cycled against insertion anodes in non-metallic,lithium ion, rocking chair cells.

The lithium mixed metal phosphate and the lithium metal oxide were usedto formulate a cathode electrode. The electrode was fabricated bysolvent casting a slurry of the treated, enriched lithium manganeseoxide, conductive carbon, binder, plasticizer and solvent. Theconductive carbon used was Super P (MMM Carbon). Kynar Flex 2801® wasused as the binder and electronic grade acetone was used as a solvent.The preferred plasticizer was dibutyl phthalate (DPB). The slurry wascast onto glass and a free-standing electrode was formed as the solventwas evaporated. In this example, the cathode had 23.1 mgLiFe_(0.9)Mg_(0.1)PO₄ active material. Thus, the proportions are asfollows on a percent weight basis: 80% active material; 8% Super Pcarbon; and 12% Kynar binder.

A graphite counter electrode was prepared for use with the aforesaidcathode. The graphite counter electrode served as the anode in theelectrochemical cell. The anode had 10.8 mg of the MCMB graphite activematerial. The graphite electrode was fabricated by solvent casting aslurry of MCMB2528 graphite, binder, and casting solvent. MCMB2528 is amesocarbon microbead material supplied by Alumina Trading, which is theU.S. distributor for the supplier, Osaka Gas Company of Japan. Thismaterial has a density of about 2.24 grams per cubic centimeter; aparticle size maximum for at least 95% by weight of the particles of 37microns; median size of about 22.5 microns and an interlayer distance ofabout 0.336. As in the case of the cathode, the binder was a copolymerof polyvinylidene difluoride (PVdF) and hexafluoropropylene (HFP) in awt. ratio of PVdF to HFP of 88:12. This binder is sold under thedesignation of Kynar Flex 2801®, showing it's a registered trademark.Kynar Flex is available from Atochem Corporation. An electronic gradesolvent was used. The slurry was cast onto glass and a free standingelectrode was formed as the casting solvent evaporated. The electrodecomposition was approximately as follows on a dry weight basis: 85%graphite; 12% binder; and 3% conductive carbon.

A rocking chair battery was prepared comprising the anode, the cathode,and an electrolyte. The ratio of the active cathode mass to the activeanode mass was about 2.14:1. The two electrode layers were arranged withan electrolyte layer in between, and the layers were laminated togetherusing heat and pressure as per the Bell Comm. Res. patents incorporatedherein by reference earlier. In a preferred method, the cell isactivated with EC/DMC solvent in a weight ratio of 2:1 in a solutioncontaining 1 M LiPF₆ salt.

FIGS. 19 and 20 show data for the first four complete cycles of thelithium ion cell having the LiFe_(0.9)Mg_(0.1)PO₄ cathode and theMCMB2528 anode. The cell comprised 23.1 mg active LiFe_(0.9)Mg_(0.1)P₄and 10.8 mg active MCMB2528 for a cathode to anode mass ratio of 2.14.The cell was charged and discharged at 23° C. at an approximate C/10 (10hour) rate between voltage limits of 2.50 V and 3.60 V. The voltageprofile plot (FIG. 19) shows the variation in cell voltage versus timefor the LiFe_(0.9)Mg_(0.1)PO₄/MCMB2528 lithium ion cell. The symmetricalnature of the charge-discharge is clearly evident. The small degree ofvoltage hysteresis between the charge and discharge processes isevidence for the low overvoltage in the system, which is very good. FIG.20 shows the variation of LiFe_(0.9)Mg_(0.1)PO₄ specific capacity withcycle number. Clearly, over the cycles shown, the material demonstratesgood cycling stability.

FIG. 21 shows data for the first three complete cycles of the lithiumion cell having the gamma-LiV₂O₅ cathode and the MCMB2528 anode. Thecell prepared was a rocking chair, lithium ion cell as described above.The cell comprised 29.1 mg gamma-LiV₂O₅ cathode active material and 12.2mg MCMB2528 anode active material, for a cathode to anode mass ratio of2.39. As stated earlier, the liquid electrolyte used was EC/DMC (2:1)and 1M LiPF₆. The cell was charged and discharged at 23° C. at anapproximate C/10 (10 hour) rate between voltage limits of 2.50 V and3.65 V. The voltage profile plot (FIG. 21) shows the variation in cellvoltage versus time for the LiV₂O₅/MCMB2528 lithium ion cell. Thesymmetrical nature of the charge-discharge is clearly evident. The smalldegree of voltage hysteresis between the charge and discharge processesis evidence for the low overvoltage in the system, which is very good.

In summary, the invention provides new compoundsLi_(a)MI_(b)MII_(c)(PO₄)_(d) and gamma-LiV₂O₅ by new methods which areadaptable to commercial scale production. The Li₁MI_(1-y)MII_(y)PO₄compounds are isostructural olivine compounds as demonstrated by XRDanalysis. Substituted compounds, such as LiFe_(1-y)Mg_(y)PO₄ show betterperformance than LiFePO₄ unsubstituted compounds when used as electrodeactive materials. The method of the invention utilizes the reducingcapabilities of carbon along with selected precursors and reactionconditions to produce high quality products suitable as electrode activematerials or as ion conductors. The reduction capability of carbon overa broad temperature range is selectively applied along withthermodynamic and kinetic considerations to provide an energy-efficient,economical and convenient process to produce compounds of a desiredcomposition and structure. This is in contrast to known methods.

Principles of carbothermal reduction have been applied to produce puremetal from metal oxides by removal of oxygen. See, for example, U.S.Pat. Nos. 2,580,878, 2,570,232, 4,177,060, and 5,803,974. Principles ofcarbothermal and thermal reduction have also been used to form carbides.See, for example, U.S. Pat. Nos. 3,865,745 and 5,384,291; and non-oxideceramics (see U.S. Pat. No. 5,607,297). Such methods are not known tohave been applied to form lithiated products or to form products withoutoxygen abstraction from the precursor. The methods described withrespect to the present invention provide high quality products which areprepared from precursors which are lithiated during the reaction withoutoxygen abstraction. This is a surprising result. The new methods of theinvention also provide new compounds not known to have been made before.

For example, alpha-V₂O₅ is conventionally lithiated electrochemicallyagainst metallic lithium. Thus, alpha-V₂O₅ is not suitable as a sourceof lithium for a cell. As a result, alpha-V₂O₅ is not used in an ioncell. In the present invention, alpha-V₂O₅ is lithiated by carbothermalreduction using a simple lithium-containing compound and the reducingcapability of carbon to form a gamma-LiV₂O₅. The single phase compound,gamma-LiV₂O₅ is not known to have been directly and independentlyprepared before. There is not known to be a direct synthesis route.Attempts to form it as a single phase resulted in a mixed phase productcontaining one or more beta phases and having the formula Li_(x)V₂O₅with 0<x≦0.49. This is far different from the present single phasegamma-Li_(x)V₂O₅ with x equal to one, or very close to one. Theflexibility of the process of the present invention is such that it canbe conducted over a wide temperature range. The higher the temperature,the more quickly the reaction proceeds. For example, at 650° C.,conversion of alpha-V₂O₅ to gamma-LiV₂O₅ occurs in about one hour, andat 500° it takes about 8 hours. Here, about one quarter (¼) atomic unitof carbon is used to reduce one atomic unit of vanadium, that is, V⁺⁵V⁺⁵to V⁺⁵V⁺⁴. The predominate reaction is C to CO₂ where for each atomicunit of carbon at ground state zero, a plus 4 oxidation state results.Correspondingly, for each ¼ atomic unit of carbon, one atomic unit ofvanadium is reduced from V⁺⁵ to V⁺⁴. (See Reaction 5). The new product,gamma-LiV₂O₅ is air-stable and suitable as an electrode material for anion cell or rocking chair battery.

The convenience and energy efficiency of the present process can also becontrasted to known methods for forming products under reducingatmosphere such as H₂ which is difficult to control, and from complexand expensive precursors. In the present invention, carbon is thereducing agent, and simple, inexpensive and even naturally occurringprecursors are useable. For example, it is possible to produce LiFePO₄from Fe₂O₃, a simple common oxide. (See Reaction 1b). The production ofLiFePO₄ provides a good example of the thermodynamic and kineticfeatures of the method. Iron phosphate is reduced by carbon andlithiated over a broad temperature range. At about 600° C., the C to CO₂reaction predominates and takes about a week to complete. At about 750°C., the C to CO reaction predominates and takes about 8 hours tocomplete. The C to CO₂ reaction requires less carbon reductant but takeslonger due to the low temperature kinetics. The C to CO reactionrequires about twice as much carbon, but due to the high temperaturereaction kinetics, it proceeds relatively fast. In both cases, the Fe inthe precursor Fe₂O₃ has oxidation state +3 and is reduced to oxidation(valence) state +2 in the product LiFePO₄. The C to CO reaction requiresthat ½ atomic unit of carbon be used for each atomic unit of Fe reducedby one valence state. The CO to CO₂ reaction requires that ¼ atomic unitof carbon be used for each atomic unit of Fe reduced by one valencestate.

The active materials of the invention are also characterized by beingstable in an as-prepared condition, in the presence of air andparticularly humid air. This is a striking advantage, because itfacilitates preparation of and assembly of battery cathodes and cells,without the requirement for controlled atmosphere. This feature isparticularly important, as those skilled in the art will recognize thatair stability, that is, lack of degradation on exposure to air, is veryimportant for commercial processing. Air-stability is known in the artto more specifically indicate that a material does not hydrolyze inpresence of moist air. Generally, air-stable materials are alsocharacterized by Li being extracted therefrom above about 3.0 voltsversus lithium. The higher the extraction potential, the more tightlybound the lithium ions are to the host lattice. This tightly boundproperty generally confers air stability on the material. Theair-stability of the materials of the invention is consistent with thestability demonstrated by cycling at the conditions stated herein. Thisis in contrast to materials which insert Li at lower voltages, belowabout 3.0 volts versus lithium, and which are not air-stable, and whichhydrolyze in moist air.

While this invention has been described in terms of certain embodimentsthereof, it is not intended that it be limited to the above description,but rather only to the extent set forth in the following claims.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined in the following claims.

1. A compound represented by the nominal formula LiFe_(1-y)Mg_(y)PO₄,wherein 0<y<1.
 2. The compound of claim 1, wherein 0<y<0.5.
 3. Thecompound of claim 2, wherein 0<y≦0.2.
 4. The compound of claim 3,wherein 0<y≦0.1.
 5. The compound of claim 1, wherein the compound has anolivine structure.
 6. The compound of claim 1, wherein the compound is asingle phase compound.
 7. The compound of claim 1, wherein y=0.2.
 8. Thecompound of claim 1, wherein y=0.1.
 9. An electrode, comprising anactive material represented by the nominal formula LiFe_(1-y)Mg_(y)PO₄,wherein 0<y<1.
 10. The electrode of claim 9, wherein 0<y<0.5.
 11. Theelectrode of claim 10, wherein 0<y<0.2.
 12. The electrode of claim 11,wherein 0<y≦0.1.
 13. The electrode of claim 9, wherein the activematerial has an olivine structure.
 14. The electrode of claim 9, whereinthe active material is a single phase compound.
 15. The electrode ofclaim 9, wherein y=0.2.
 16. The electrode of claim 9, wherein y=0.1. 17.The electrode of claim 9, further comprising an electrically conductivediluent, and a binder.
 18. The electrode of claim 17, wherein theelectrically conductive diluent is carbon.
 19. The electrode of claim18, wherein the electrically conductive diluent is carbon black.
 20. Theelectrode of claim 19, wherein the electrode comprises from 5 to 30% byweight carbon black.
 21. The electrode of claim 17, wherein the binderis a copolymer of polyvinylidene difluoride (PVdF) andhexafluoropropylene (HFP).
 22. The electrode of claim 17, wherein theelectrode comprises from 3 to 20% by weight binder.
 23. A battery,comprising: a first electrode comprising an active material representedby the nominal formula LiFe_(1-y)Mg_(y)PO₄, wherein 0<y<1; a secondelectrode which is a counter-electrode to the first electrode; and anelectrolyte.
 24. The battery of claim 23, wherein 0<y<0.5.
 25. Thebattery of claim 24, wherein 0<y≦0.2.
 26. The battery of claim 25,wherein 0<y≦0.1.
 27. The battery of claim 23, wherein the activematerial has an olivine structure.
 28. The battery of claim 23, whereinthe active material is a single phase compound.
 29. The battery of claim23, wherein y=0.2.
 30. The battery of claim 23, wherein y=0.1.
 31. Thebattery of claim 23, wherein the first electrode further comprises anelectrically conductive diluent, and a binder.
 32. The battery of claim31, wherein the electrically conductive diluent is carbon.
 33. Thebattery of claim 32, wherein the electrically conductive diluent iscarbon black.
 34. The battery of claim 33, wherein the first electrodecomprises from 5 to 30% by weight carbon black.
 35. The battery of claim31, wherein the binder is a copolymer of polyvinylidene difluoride(PVdF) and hexafluoropropylene (HFP).
 36. The battery of claim 31,wherein the first electrode comprises from 3 to 20% by weight binder.37. The battery of claim 23, wherein the second electrode comprises aninsertion active material.
 38. The battery of claim 37, wherein theinsertion active material is selected from the group consisting of ametal oxide, metal chalcogenide, carbon, graphite, and mixtures thereof.39. The battery of claim 37, wherein the insertion active material isgraphite.
 40. The battery of claim 37, wherein the first and secondelectrodes each further comprise an electrically conductive diluent, anda binder.
 41. The battery of claim 40, wherein the electricallyconductive diluent is carbon.
 42. The battery of claim 41, wherein theelectrically conductive diluent is carbon black.
 43. The battery ofclaim 42, wherein the first and second electrode each comprise from 5 to30% by weight carbon black.
 44. The battery of claim 40, wherein thebinder is a copolymer of polyvinylidene difluoride (PVdF) andhexafluoropropylene (HFP).
 45. The battery of claim 40, wherein thefirst and second electrode each comprise from 3 to 20% by weight binder.46. The battery of claim 23, wherein the electrolyte comprises a lithiumsalt and a solvent selected from the group consisting of dimethylcarbonate (DMC), diethylcarbonate (DEC), dipropylcarbonate (DPC),ethylmethylcarbonate (EMC), ethylene carbonate (EC), propylene carbonate(PC), butylene carbonate, lactones, esters, glymes, sulfoxides,sulfolanes, and mixtures thereof.